<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>示例14</title>
</head>
<body>
    <script>
        const map = new Map()
        map.set("foo",789)
        map.set("bar",true)
        map.set(undefined,404)
        console.log(map)
        console.log(map.size)
        console.log(map.has("foo"))
        console.log(map.get("foo"))
        console.log(map.get(undefined))
    </script>
</body>
</html>